Vertically movable sectional doors, such as those used in warehouses and other industrial settings typically employ spring, pulley and cable assemblies to counterbalance the weight of the overhead door in its motion along roller tracks.
Any imbalance which arises, e.g. through aging of the component springs of the door mechanism can result in an undesired and unpredictable tendency of the overhead door to “coast” or “drift”. This can have costly and dangerous consequences, for example, in the event of a downward drifting of an opened sectional door as a forklift truck is backing up through the opening.
A number of systems have been devised to eliminate door drift, typically making use locking devices that are mechanically or electrically switchable between on and off positions. Examples are afforded by the drop-catch mechanism for vertically movable doors disclosed in U.S. Pat. No. 6,042,158 (Horn) and the locking system for sectional doors disclosed in U.S. Pat. No. 6,145,570 (Mullet et al).
None of these systems, however, allows one to manually raise or lower the door in the usual way to a selected position and then leave the door there, with confidence that it will not drift up or down from that selected position.